1. Field of the Art
The disclosure relates generally to communication systems, and more specifically, to cycle slip compensation in a coherent receiver.
2. Description of the Related Art
The most recent generation of high-speed optical transport network systems has widely adopted receiver technologies with electronic dispersion compensation (EDC). These receivers often use phase encoded modulation formats such as phase shift keying (PSK) or quadrature PSK (QPSK). Carrier phase recovery (CPR) is a key function of coherent optical receivers. CPR algorithms compensate for effects such as laser phase noise and carrier frequency fluctuations. However, since various modulation schemes such as QPSK have rotational symmetry, errors in the carrier phase estimation may cause cycle slips. After a cycle slip occurs, all detected symbols are erroneous and they generally cannot be corrected by forward error correction (FEC) codes. To combat this catastrophic effect, differential modulation is typically used. In differential modulation schemes the information is transmitted as the phase difference between two consecutive symbols. Therefore, the effects of a cycle slip do not translate into catastrophic bit errors when differential modulation is used. However, differential modulation is prone to introduce signal-to-noise ratio (SNR) penalty compared to non-differential schemes, and is therefore an undesirable solution to the cycle slip problem.